This invention relates to an optical information recording material of the type having a light-absorbing film which exhibits a change in optical density when light such as a laser beam is incident thereon and a method of optically recording and reproducing information by using the same recording material.
As the quantities of information to be recorded and reproduced are progressively increasing in recent years, there is a keen demand for an innovative method that enables more high-speed and more high-density recording of information and also more high-speed reproduction of the recorded information than the currently practiced information recording and reproducing methods. In this regard, much attention has recently been given to various methods of optically recording and reproducing information. Typical examples of hitherto proposed methods of this category are as follows.
As the first example, a recording method reported by D. Maydan, "The Bell System Technical Journal", Vol. 50, No. 6(1971), pp. 1761-88, is the use of a thin film (about 0.1 .mu.m) of a low melting point metal such as indium or bismuth formed on a glass or plastic substrate. Using a laser beam, illumination of the metal thin film in very tiny spots (about 10 .mu.m or smaller in diameter) causes formation of tiny cavities in the illuminated spots through absorption of light, rise in temperature and resultant fusion, displacement and/or evaporation of the metal. Thus the illuminated spots become transparent or translucent, so that an optically reproducible and permanent image is produced. A particular advantage of this method is the possibility of producing images of considerably high contrast ratio. However, limitations are placed on the construction of recording materials according to this method because of the occurrence of displacement of the material at the changes in optical density of the metallic recording film. Besides, it offers a problem in practical applications that the recording sensitivity of the metallic film is as low as 10.sup.3 -10.sup.2 mJ/cm.sup.2.
As the second example, U.S. Pat. No. 3,636,526 proposes the use of an amorphous semiconductive material, more particularly a calcogenide not containing oxygen as typified by Ge.sub.15 Te.sub.81 Sb.sub.2 S.sub.2, As.sub.2 S.sub.3 and As.sub.20 Se.sub.60 Ge.sub.20. When light such as a laser beam is applied to a thin film of such material, there occurs a change in the bonding state of the atoms of the material as a result of temperature increase by absorption of light, so that the thin film exhibits a change in optical density. As an advantage of this recording material, it is possible to erase the recorded information by applying light of adequate intensity. However, this recording material requires a recording light energy density of 2.times.10.sup.2 mJ/cm.sup.2 or above and suffers insufficient contrast ratio.
A still another method is the use of a thin film of a suboxide of a metal or semimetal. For example, U.S. Pat. No. 3,971,874 proposes a recording material having a thin film of which essential material is TeO.sub.x where x is smaller than 2.0 When irradiated with light such as a laser beam, the thin film of the suboxide undergoes a change from a low optical density state to a high optical density state as a result of temperature increase by absorption of light energy. This optical recording material is advantageous in that the recording sensitivity is very high (10.sup.2 mJ/cm.sup.2 or below) and that the magnitude of a maximum change in the optical density is very large (more than 30%) so that recording can be made with a very high contrast ratio (greater than 10:1). However, compared with the above described amorphous calcogenide this recording material is less easy of erasing information recorded thereon.